From the dawn of human civilisation until sometime in the
nineteenth century, cities were net population
sinks—the increased mortality from infectious
diseases, compounded by the unsanitary conditions,
impure water, and food transported from the hinterland
and stored without refrigeration so shortened the lives
of city-dwellers (except for the ruling class and the
wealthy, a small fraction of the population) that a city's
population was maintained only by a constant net migration
to it from the countryside. In densely-packed cities, not
only does an infected individual come into contact with many
more potential victims than in a rural environment, highly
virulent strains of infectious agents which would
“burn out” due to rapidly killing their hosts
in farm country or a small village can prosper in
a city, since each infected host still has the opportunity
to infect many others before succumbing. Cities can be
thought of as Petri dishes for evolving killer microbes.

No civic culture medium was as hospitable to pathogens
as London in the middle of the 19th century. Its population,
2.4 million in 1851, had exploded from just one million
at the start of the century, and all of these people had been
accommodated in a sprawling metropolis almost devoid of
what we would consider a public health infrastructure.
Sewers, where they existed, were often open and simply
dumped into the Thames, whence other Londoners drew
their drinking water, downstream. Other residences
dumped human waste in cesspools, emptied
occasionally (or maybe not) by “night-soil men”.
Imperial London was a smelly, and a deadly place.
Observing it first-hand is what motivated
Friedrich Engels to document and deplore
The
Condition of the Working Class in England
(January 2003).

Among the diseases which cut down inhabitants of
cities, one of the most feared was cholera. In 1849,
an outbreak killed 14,137 in London, and nobody knew when
or where it might strike next. The prevailing theory of disease
at this epoch was that infection was caused by and spread
through
“miasma”:
contaminated air. Given how
London stank and how deadly it was to its inhabitants,
this would have seemed perfectly plausible to people
living before the
germ
theory of disease was propounded.
Edwin Chadwick,
head of the General Board of Health in London
at the epoch, went so far as to assert (p. 114)
“all smell is disease”. Chadwick was, in
many ways, one of the first advocates and implementers
of what we have come to call “big government”—that
the state should take an active role in addressing social
problems and providing infrastructure for public health.
Relying upon the accepted “miasma” theory and
empowered by an act of Parliament, he spent the 1840s trying
to eliminate the stink of the cesspools by connecting them to
sewers which drained their offal into the Thames. Chadwick was,
by doing so, to provide one of the first demonstrations of
that universal concomitant of big government,
unintended consequences:
“The first defining act of a modern, centralized public-health
authority was to poison an entire urban population.”
(p. 120).

When, in 1854, a singularly virulent outbreak of cholera
struck the Soho district of London, physician and pioneer
in anæsthesia
John Snow
found himself at the fulcrum of a
revolution in science and public health toward which he had
been working for years. Based upon his studies of the 1849
cholera outbreak, Snow had become convinced that the pathogen
spread through contamination of water supplies by the excrement
of infected individuals. He had published a monograph laying
out this theory in 1849, but it swayed few readers from the
prevailing miasma theory. He was continuing to document the
case when cholera exploded in his own neighbourhood. Snow's
mind was not only prepared to consider a waterborne infection
vector, he was also one of the pioneers of the emerging science
of epidemiology: he was a founding member of the
London Epidemiological Society in 1850. Snow's real-time analysis
of the epidemic caused him to believe that the vector of infection
was contaminated water from the Broad Street pump, and his
persuasive presentation of the evidence to the Board of Governors
of St. James Parish caused them to remove the handle from that
pump, after which the contagion abated. (As the
author explains, the outbreak was already declining at the time,
and in all probability the water from the Broad Street pump was
no longer contaminated then. However, due to subsequent
events and discoveries made later, had the handle not been
removed there would have likely been a second wave of the
epidemic, with casualties comparable to the first.)

Afterward, Snow, with the assistance of initially-sceptical
clergyman Henry Whitehead, whose intimate knowledge of the
neighbourhood and its residents allowed compiling the data
which not only confirmed Snow's hypothesis but identified what
modern epidemiologists would call the “index case”
and “vector of contagion”, revised his monograph
to cover the 1854 outbreak, illustrated by a map which illustrated
its casualties that has become a classic of on-the-ground
epidemiology and the graphical presentation of data. Most
brilliant was Snow's use (and apparent independent invention) of
a Voronoi
diagram to show the boundary, by streets, of the distance,
not in Euclidean space, but by walking time, of the
area closer to the Broad Street pump than to others in the
neighbourhood. (Oddly, the complete map with this crucial
detail does not appear in the book: only a blow-up of the central
section without the boundary. The
full
map is here; depending on your browser, you may have to click on
the map image to display it at full resolution. The dotted and dashed
line is the Voronoi cell enclosing the Broad Street pump.)

In the following years, London embarked upon a massive program
to build underground sewers to transport the waste of its millions
of residents downstream to the tidal zone of the Thames and later,
directly to the sea. There would be one more cholera outbreak in
London in 1866—in an area not yet connected to the new
sewers and water treatment systems. Afterward, there has not
been a single epidemic of cholera in London. Other cities in the
developed world learned this lesson and built the infrastructure
to provide their residents clean water. In the developing world,
cholera continues to take its toll: in the 1990s an outbreak in South
America infected more than a million people and killed almost
10,000. Fortunately, administration of
rehydration therapy
(with electrolytes) has drastically reduced the likelihood of
death from a cholera infection. Still, you have to wonder why,
in a world where billions of people lack access to clean water
and third world mega-cities are drawing millions to live in
conditions not unlike London in the 1850s, that some believe
that laptop computers are the top priority for children growing up
there.